Shape similarity between solute and solvent molecules: its role in specific molecular recognition in hydrocarbon-containing solvents

Oxidation of a pair of associating thiols (1 and 2), each having a binding site [–C(O)NHC(O)NH–] and a recognition site (R1 or R2), is examined mainly in hydrocarbon-containing (aqueous) alcohols. The selectivity (r), a measure of the degree of recognition of 1 by 2(or of 2 by 1), in the oxidation is defined as the logarithmic ratio of the yield of an unsymmetrical disulfide to twice that of a symmetrical one. It is found that (i) higher selectivity is achieved when a hydrocarbon similar in shape to R2 is present in (aqueous) ethanol and (ii) the selectivity for R2= Ph (= R1) is higher in pure benzene than in aqueous ethanol. Correlation of the observed selectivity with physicochemical properties (relative permittivity and viscosity) of the pure solvents, reactivity difference between thiols, and other factors is discussed, together with a possible explanation of the specific recognition in terms of a ‘shape similarity effect’.